This invention provides a current controlled AC/DC power supply for arc lamps, such as HMI lamps, mercury vapor lamps, sodium vapor lamps, and the like. Such lamps are used in theatrical productions, on cinematographic stages, for the production of TV shows, in industrial applications, for lighting sporting events and for street and outdoor lighting in general, to name only a few applications. When used in cinematographic, theatrical, and TV applications and sometimes when used in industrial or sporting applications, the lamp selected must have a correct light spectrum characteristic (or color temperature), which often means that it must have the same light spectrum (color temperature) as the sun so that colors appear natural. In such cases, the current supplied by the power supply to the lamp must be carefully controlled in order to provide precise regulation of lamp color temperature.
The prior art power supplies have a number of drawbacks. They tend to be bulky, expensive and slow to ignite or re-ignite the lamp being powered. In the case of power supplies used for theatrical work, cinematographic work or in a TV studio, the power supplies should be preferably portable (and, in the case of the prior art, they were, at best, semi-portable because the power supplies were quite heavy), they should generate flicker free light and need to be able to ignite and re-ignite the lamp quickly. In industrial applications, the ability to re-ignite a lamp quickly can also be very important. In an effort to save energy costs, many factories have switched from traditional lamps to arc lamps due to their greater energy efficiency. Power outages can occasionally occur, however, and since the power supplies used to power such lamps have required a comparatively long time to restart or re-ignite the lamps, the factory can be without light for a considerable length of time. If it takes more than a few minutes to restart the lamps, then the loss of production at the factory can outweigh the savings from using such lamps.
HMI lamps and other similar lamp types should not be operated on DC (Direct Current) because DC causes erosion of the electrodes resulting in rapid destruction of the bulb. If the lamp is powered by a sinusoidal AC (Alternating Current) waveform, the erosion problem is overcome, but the resulting light emitted varies sinusoidally resulting in the phenomenon known as flicker. Those skilled in the art realize that flicker is undesirable, especially in cinematographic applications. If a square waveform is utilized to power the lamp then both the erosion problem and the flicker problem are overcome. U.S. Pat. No. 4,485,434 teaches how to generate a squarewave using a bridge circuit.
Readily available power sources supply sinusoidal AC. For example, the 120 volt 60 Hertz power available in American homes and industry is sinusoidal AC. On cinematographic stages DC power has traditionally been available. It is well known to convert sinusoidal AC to DC by means of a simple rectifier and filter device. A converter device is then used to obtain the proper current and voltage for the lamp.
Preferably, a power supply for arc lamps should be able to be powered from either AC or DC sources of wide voltage ranges. In particular, the power supply should be functional even when the input voltage is less than the voltage required to ignite and run the arc lamp. Moreover, the power supply should be light weight, cost effective to manufacture and yet provide sufficient power resources to quickly ignite or re-ignite the arc lamp.